In duplex document scanning applications, it is desirable to merge both front and rear document image acquisition system outputs into a single composite image. It is further desirable to merge the outputs of two image acquisition systems, which are asynchronous to one another. Generally speaking, the image data from the two asynchronous data streams could be interleaved in one of three possible ways, 1) pixel interleaving where each pixel could be n-bits wide, 2) raster interleaving where a raster is comprised of several n-bit bytes, and 3) document interleaving where a document is comprised of several rasters.
Merging data from multiple sources can be accomplished in a variety of ways. The first method requires that the data streams be synchronous to one another. For example, one method might implement a master clock on one image acquisition system while the other image acquisition system is synchronized to the master via a phase locking mechanism. From a manufacturing perspective, this requires that two different parts be stocked; a master and a slave. Furthermore, group delays associated with transmitting the master clock between systems would cause a skew between the data and clock of the slave system. This is especially true, and a potential limitation, at high data rates or while transmitting data over long distances.
Another scheme for merging data from multiple sources is to have a master clock at the receiver. This master clock is then transmitted to the image acquisition systems. Once again, this scheme presents a problem with group delays which would cause skew between the data and the clock, possibly limiting data rates and transmission distances. Furthermore, the clock frequency of the image acquisition systems in this scenario is dictated by the data receiver making the system less flexible.
The method described in U.S. Pat. No. 6,115,377 describes a method wherein the image acquisition systems are allowed to be asynchronous and the receiver merges the data by buffering the data. A problem with this method results from the fact it uses the document length for calculating the image frame size, which is used to calculate the total additional memory requirements (TAMR). This limits the document length to the amount of memory available. Furthermore, it uses the image acquisition system (IAS) offset and therefore places restrictions on the offset between the two streams to be merged. This method requires a moment in time where both data streams are idle or contain invalid data to prevent the buffers from overflowing.
It is desirable to allow each image acquisition system, or any other data source, to select a data rate that is appropriate for the application. It is also desirable to allow each system to supply its own clock, completely asynchronous to one another. Finally, it is desirable to synchronize and merge the data from the sources without concern for the length of the data stream or location of valid data.